Electrophoretic display drive techniques

ABSTRACT

Techniques for updating an electrophoretic display or other integrating display are provided. A first image data representing the current optical state of the display is combined with a second image data representing the desired next optical state of the display along with a third data representing the charge history of the display to form a compact intermediate representation of the electrical drive signals required to transition the display. Such compact intermediate representations can be provided for each pixel of the display and stored in flash memory. Once determined, these representations of the drive signals can be rapidly replayed from flash memory and further processed to drive the display and affect the desired optical transition from current image to next image while correcting for DC imbalances.

FIELD OF THE DISCLOSURE

The present application relates to display technology, and morespecifically to integrating displays such as electrophoretic displays.

BACKGROUND

Reflective electrophoretic displays (EPDs) are used in a number ofapplications where common emissive display technologies such as liquidcrystal displays (LCDs) exhibit limitations. The most popular types ofEPDs have two key distinctions compared to the more common continuouslyrefreshed LCDs and organic light emitting diode (OLED) displays, namelythat they are that they are reflective (rather than emissive), and canalso maintain optical state without any applied electrical drive energy.Combined, these two properties give EPDs an advantage in brightenvironments for applications that show mostly long-dwell content (e.g.,electronic books or so-called eBooks, and price labels or other digitalreadouts) and benefit from low power consumption.

The most common application for large EPDs is currently electronic bookreaders, although there are other applications as well, such aselectronic signage. The canonical architecture for an EPD e-reader istypically comprised of a system-on-chip (SoC) running a Linux-basedoperating system (e.g., Android), connecting over WiFi and driving anEPD. One architectural limitation of EPD based e-readers is aconsequence of the EPD's physical response to electrical stimuli whichnecessitates an image update process that is very different fromcontinuously refreshed LCDs. There are two particular implications withrespect to this architectural limitation. First, EPDs are generallyrelatively slow to update; to achieve a pixel transition from a blackstate to a white state may take 250 milliseconds or more, in contrast toLCDs which often respond within 10's of milliseconds or faster. Second,most EPDs exhibit a memory effect, i.e., once the electrical stimulus isstopped the black/gray/white state of the EPD material or so-calledelectronic ink is approximately static. The EPD effectively integratesthe time-varying electrical stimuli provided by the driving chips anddisplay system to change from a known starting optical state to adesired end optical state. As such, an EPD display is referred to hereinas one example of an integrating display.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates block diagram of a first comparative EPD drivingsystem based on an EPD controller ASIC and external random access memory(RAM) configuration.

FIG. 2 illustrates a block diagram of a second comparative EPD drivingsystem based on a SoC with a software or embedded EPD controller andexternal RAM configuration.

FIG. 3 illustrates a block diagram of an EPD driving system configuredto utilize flash memory as both the content source and working memoryfor the EPD image update process, in accordance with an embodiment ofthe present disclosure.

FIG. 4 illustrates a flowchart of an EPD image update process, inaccordance with an embodiment of the present disclosure.

FIG. 5 shows a representative example of an image and charge data flowfor calculating a drive signal for the calculating process shown in FIG.4, in accordance with an embodiment of the present disclosure.

FIGS. 6a-c collectively show a representative example of the displayupdate process shown in FIG. 4, in accordance with an embodiment of thepresent disclosure.

DETAILED DESCRIPTION

Techniques are disclosed for efficiently and quickly updatingelectrophoretic displays (EPD), and other such integrating displays thatintegrate a time-varying drive signal to change from a current opticalstate to a next optical state. In an embodiment, a display driver systemis configured to update an EPD using a drive signal calculation phasefollowed by an update display phase. The calculation phase effectivelydecomposes the image update into a number of compactly encodedintermediate files, and the subsequent update display phase replays thecalculated files repeatedly and sequentially to update the display. Thecompact intermediate files are representations of the drive signalsneeded to effect the display update and can be implemented in a numberof ways. In one example case, a first image data representing thecurrent optical state of the display for a given pixel position iscombined with a second image data representing the next optical state ofthe display for that pixel position, thereby forming an index into awaveform (drive signal) look-up process. In another example case, theindex is formed by further combining the current and next image pixeldata with a third data representing the charge history of that pixel. Inany such cases, the output of the waveform look-up process yields amulti-pass drive signal that transitions the corresponding pixel fromthe current image to the next image. This calculation is done for eachpixel, and the resulting intermediate files are stored in a memory.During the subsequent update display phase, these representations of thetarget drive signals are rapidly replayed from the memory and furtherprocessed to drive the display and cause the desired optical transitionfrom current image to next image while correcting for direct current(DC) imbalances. A consequence of the this multi-pass process is that itcan be used to efficiently and quickly update a high resolutionintegrating display such as an EPD using low cost memory components suchas flash memory, rather than relatively more expensive memory. As willbe further appreciated in light of this disclosure, the multi-passprocess can be used to offload the computational requirements such thatvery low cost and power efficient EPDs can be produced.

Overview and Context

Before further explaining the display drive techniques provided herein,it may be helpful to first review some of the various possible EPDdriving techniques and various features associated with EPD technologyto give proper context. Such review may be particularly helpful inassisting the reader to better appreciate the nuance of the techniquesprovided in the present disclosure.

While EPDs can respond to multilevel (e.g., 256 levels, or 8 bit) analogvoltages to achieve grayscale, it is prudent for cost reasons to apply atime-modulated three level voltage (+V, 0, −V) to EPDs as a quicksuccession of three-levels-per-pixel image frames to achieve multiplegray levels. Typical EPD material response is sufficiently slow (e.g.,250 ms transition time) compared to easily achievable SoC displaydriving hardware frame rates (e.g., 85 Hz, which is approximately 12 msper frame) that enough gray levels (e.g., 8 to 16) can be achieved torender anti-aliased text well (e.g., 250 ms/12 ms≈21 distinct raw levelsof varying gray shades, sufficient to support 3-bpp or 4-bpp grayscaleimages conceptually). As a consequence of the EPD physical response, themost common driving architecture for EPD e-readers uses a relativelyhigh frame rate display controller (e.g., 85 Hz) to provide aframe-by-frame varying three-level-per-pixel image change sequenceencoded with 2 bits per pixel (bpp) per frame. Any image areas that areto be left unchanged may repeatedly receive 0V stimuli for the entiremulti-frame sequence—only pixels that are changing gray state arerequired to be driven at +V or −V for one or more frames.

An additional constraint on the EPD driving process is due to theintegrating nature of the EPD material itself; in contrast to LCDdevices, whose state is directly related to the currently appliedvoltage with little or no history effect, EPDs are typically engineeredwith a strong memory effect so that they will retain their optical stateonce the driving waveforms are discontinued. In order to drive from onegray state to another, the EPD controller must make an assumption aboutthe initial state of the pixel, the last image shown, for example.However, any cumulative grayscale imperfections arising from thequantized nature of the drive (i.e., some integer number of frameperiods at +V, −V or 0V), the manufacturing variance of the EPDmaterial, its driving history, and/or a temperature measurement orresponse error, among other things, will result in an imperfectpost-drive optical state. To counter the effect of such intrinsic and/oraccumulating pixel grayscale errors, the EPD material can be driven tooptical saturation (e.g., a nearly full black or white state) wheredriving it harder into a black or white rail will have a negligibleeffect on the grayshade. This allows the driving system to better knowthe pixel state with reasonable certainty; using such an overdriveperiodically at the start of a frame change can counter grayscale erroraccumulation. However, such drive sequences (driving the whole image ora significant fraction of the pixels to a full black or white statebefore updating them to their final desired state) is noticeable tousers as a flashing transition. Such clearing or resetting transitionsare generally slower than direct transitions since the EPD must firsttransit to a full black or white state, then additionally transition tothe desired final grayscale state. This presents a tradeoff—one canchoose more frequent clearing/resetting which will result in improvedgrayscale accuracy at the expense of flashing and transition time, oralternatively, one can choose making fewer clears/resets which willspeedup response but lead to an accumulation of pixel errors. Since theerrors are most pronounced at changing pixels, the pixel errors oftenresemble prior displayed images; for this reason these cumulative pixelserrors are termed ghosting.

The process of changing from a current image to a new image on an EPDcomprises the steps of first computing the difference between thecurrent optical state of the EPD (e.g., the last image shown) and thenew image to be shown. In the most commercially significantimplementations, the image difference is then used to look-up aper-pixel time-sequence of two bit values (selecting one of +V, −V or 0voltage states) that is then driven to the EPD as a time sequence offrames to cause the image transition. The look-up table encodes thesequence of 2-bit per pixel (2-bpp) values for generating the desiredchange in optical state of the EPD; these waveform look-up tables areoften empirically tuned per manufacturing batch of EPD material toachieve accurate and/or fast grayscale transitions. In practice, thereare additional effects captured in the look-up table for a given EPD.EPD transition times are strongly temperature dependent. As such,waveform look-up tables are usually provided for a number of differenttemperature ranges with unique tables for temperature differences assmall as a few degrees C. Additionally, waveforms may be selectivelyoptimized for transition speed at the expense of grayscale accuracy, forexample, resulting in additional tables.

In addition to encoding the sequence of +V, −V and 0 voltages to cause agiven grayscale change, the waveform tables may also account for the DCbalance of the EPD material. If repetitively driven with an imbalancedwaveform (such as the example case where +V is driven more often than −Vover long time scales), the EPD material will suffer long term changesto its response characteristic often leading to a phenomena known aspermanent ghosting, image burn-in, and/or image sticking where aresidual image corresponding to the long term DC imbalance is faintlyvisible, blended with the desired image. To prevent such image stickingphenomena, waveform tables are often constructed to be directly DCbalanced, wherein each transition from one gray level to another isexplicitly constrained to be DC balanced, so that all transitioncombinations (from any grayscale level to any other grayscale level) areintrinsically balanced. Alternatively, a transition can be constructedto DC compensate for the last transition's ending, thus assuring (overthe last transition's end and the current transition's beginning) thatDC balance is maintained. These types of constructions, while guaranteedsafe for the panel from a DC perspective, can have negative sideeffects. For instance, direct DC balance of every transition, pair oftransitions, or pair of partial transitions can negatively impacttransition time, flashing, and/or optical accuracy of the EPD.

In contrast to simply refreshed displays such as LCDs, the EPD drivesignals are relatively complex to determine, in that EPDs require morethan the new frame information. In more detail, the calculation of therequired sequence of 2-bpp frames typically should take into account atleast the following variables for decent grayscale control as requiredby e-readers: the previous image, the desired new image, the temperatureof the EPD, and the desired transition type (e.g., fast and inaccurate,clearing/setting, slow and accurate, to name a few possible examplescenarios). Historically, the determination of these waveforms or drivesignals was performed in a dedicated hardware subsystem consisting of anEPD application specific integrated circuit (ASIC) and a dedicated framebuffer memory. FIG. 1 shows an example of one such architecture, andwill be discussed in turn. With the proliferation and cost reduction ofhigh speed SoCs used in mobile phones, the most common architecture hasevolved to a more software-based approach wherein a software blockcompiles the various inputs into a sequence of 2-bpp frames that aretemporarily stored in main system memory (typically external dynamicrandom access memory, or DRAM) that are then sequentially scanned to theEPD through an SoC video port to affect the desired image change. FIG. 2shows an example of this architecture, and will be discussed in turn.Note that the particular configurations of FIGS. 1 and 2 are notnecessarily provided as examples of conventional architecture; rather,they are provided as comparative architectures.

FIG. 1 shows a block diagram of an example EPD driving system based onan EPD controller ASIC. In operation, the system-on-chip (SoC) 100renders a new image for display using content stored in flash memory 105and stores this image in its external DRAM memory 101 in a simpleserialized array data structure. The SoC 100 will then provide this newimage to the EPD controller ASIC 102 which stores the image and/or oneor more processed versions of the image temporarily in a second externalRAM 103. The EPD controller ASIC 102 has access to a waveform datastructure containing look-up tables that capture the particular EPDdisplay 104 characteristics by providing a sequence of pixel drivingstates (one of +V, −V or 0 volts) to affect the desired change in pixeloptical state while preserving DC charge balance. To initiate an imagetransition, the SoC 100 sends a command to the EPD controller ASIC 102that triggers both the computation of a specific sequence of frames aswell as the output drive of the specific sequence of frames. In astandby power state, SoC 100 enters a low power idle state after storingcritical state information into the external DRAM memory 101 and puttingthat memory 101 into a low powered self-refresh mode. Other majorcomponents including power management 106, battery 107 and a wirelessconnectivity subsystem 108 (e.g., WiFi or BlueTooth) can be used tofurther round out the system.

FIG. 2 illustrates a block diagram of an EPD driving system based on anSoC with a software or embedded EPD controller. In operation, the SoC200 renders a new image for display from content stored in flash memory203 and stores this image temporarily in external DRAM memory 201. TheSoC 200 will then trigger the calculation of 2-bpp frames based on thecurrent image, the desired new image and waveform data for theparticular EPD at a given temperature and place the compiled 2-bpp framesequence(s) into the external DRAM 201 temporarily until they are drivento the EPD 202. Similarly to the system of FIG. 1, the standby powerstate is achieved when the SoC 200 enters a low power idle state afterstoring critical state information into the external DRAM memory 201that is subsequently placed in a low powered self-refresh state. Powermanagement block 206, battery 205 and wireless subsystems 204 can beused to further supplement the system.

Note that additional elements and components not shown (e.g., buttons,touch screen, battery and battery gas gauge, audio subsystem, frontlights, to name a few) may be desirable features in an e-reader. As willbe appreciated, some embodiments of the present disclosure are primarilyconcerned with the major components that play a role in the driving ofthe EPD or overall system function related to standby power. To thisend, and as will be further appreciated, the addition of such additionalor accessory elements to the EPD driving system architecture does notfundamentally change or limit the present disclosure to any particularconfiguration or architecture scheme.

In any such cases, despite the relatively slow speed of the EPD imagetransitions, the data throughput demands are surprisingly high. Duringcompilation, two frames of image data must be combined to make an output(as contrasted with just one for LCDs), and during EPD driving, the SoCsubsystem must provide a sequence of potentially unique 85 Hz frames tothe EPD display (as contrasted with typical 60 Hz frames for LCDs, whererepeated frames can often be tolerated). The lower number of grayscales(e.g., 16 grayscales contrasted with 16 M colors for 24-bpp LCDs) doesmitigate the computational load somewhat, but the high frame rate,unique and precise sequencing and multiple input frames requirenon-trivial compute and memory resources (size and speed). When combinedwith other functions typically required of e-readers (e.g., establishingconnectivity, procuring and decrypting digital content, rendering pages,implementing responsive user interfaces, dictionaries, search, librarysynchronization, etc.), the typical system architecture requires adecently efficient processor core running at nearly smartphone-likespeeds to give a good user experience. Rounding out the typical system,non-volatile storage (e.g., flash memory) is required to store theoperating system (e.g., Android or Linux-based), applications (e.g., thereader application), and digital content (e.g., ePubs, PDFs, etc.).

As a consequence of this computational load, the conventional EPD readerarchitecture has a mobile phone scale processor along with a mobilephone sized battery (e.g., 1000 mAh to 1500 mAh lithium polymer batterypack). The digital subsystems, especially the SoC and RAM, consumesignificant power compared to what is actually needed by an EPD device(which is effectively zero power in between image updates) for averagereader usage patterns, and as will be appreciated in light of thisdisclosure. Furthermore, due to the complexity and pedigree of theoperating systems commonly used pursuant to mobile phone heritage(typically, Linux, Android or other such robust operating systems),shutting the operating system or so-called OS completely off to savepower in standby incurs a significant reboot time (e.g., 30 seconds froma cold start for many current e-readers). In order to be responsive to auser's startup request (e.g., within a few seconds of pushing the powerbutton), it is common to put the SoC into a nonzero power standby statewith the SoC idling and the RAM in a low power refresh mode. Aconsequence of this (relatively) fast wake feature and large OS is thatthe power consumption of the typical e-reader device in standby isactually quite high. Thus, it is typical to budget a significant(oftentimes the dominant) portion of the battery capacity to standby(inactive) operation.

Thus, and in accordance with an embodiment of the present disclosure, anEPD drive system is provided that minimizes or otherwise reduces theamount of computation and memory required in the device to save powerand cost. The system may be implemented, for example, as a highresolution EPD drive system that can update the EPD directly from flashmemory and does not require external RAM-based frame buffers, despitethe severe bandwidth limitations that flash memory exhibits compared toRAM. In some embodiments, the EPD drive system may be further configuredto with a DC charge balance monitoring and correcting system, and/or asystem for optimizing drive waveforms using EPD specific empirical dataand an offline optimization pass. Numerous other embodiments andvariations will be apparent in light of this disclosure.

Display Driver Architecture

FIG. 3 illustrates a block diagram of an EPD driving system configuredin accordance with an embodiment of the present disclosure. As can beseen, this particular example system includes a SoC 300, a systemcontroller 301, flash memory 302, an EPD 303, a wireless connectivitysubsystem 304, a battery 305, and a power management block 306. Otherexample embodiments may have fewer or more modules, as will beappreciated. For instance, another example case may be implementedwithout the wireless connectivity subsystem 304 and the power managementblock 306. In addition, some modules shown as distinct and separate mayalternatively be integrated with other modules, such as the controller301 may be integrated with the SoC 300, and the flash memory 302 may beembedded in the controller 301. Also, although the techniques providedherein are particularly useful for mobile computing devices such ase-readers, tablets, and smartphones that are power constrained, there isno reason why the techniques provided herein cannot be used inconjunction with wired computing systems (no battery) as well. To thisend, the EPD driving system depicted in FIG. 3, and further demonstratedin FIGS. 4-6 a-c, can be included in or otherwise used with any type ofcomputing device or system having an EPD or other integrating displaythat can benefit from the driving techniques provided herein.

The system of FIG. 3 is capable of updating the EPD 303 using only smallkilobyte size memories (e.g., RAM or other suitable onboard memory)internal to both the SoC 300 and controller 301 by utilizing the flashmemory 302 as both the display content source and working memory for thedisplay update. To this end, note the memory 302 includes image data(e.g., current image and next image data structures), charge history(C.H.) data, and LUTs containing low bit count pulse width modulation(PWM) indices, generally referred to herein as playback files, and aswill be explained in turn. Thus, no external DRAM (101, 201) is neededfor the display update process.

Flash memory is generally much slower than RAM for both read and writeoperations. Flash memory is engineered for very high bit density (muchhigher than RAM, generally) and as a consequence has both a much lowercost per bit and much slower access time. For example, while DRAMs suchas 101 and 201 of the respective systems shown in FIGS. 1 and 2 may berandomly accessed in 10's to 100's of nanoseconds, flash memory may take10's to 100's of microseconds. Flash memory access is also highlyasymmetric, in that write access is often significantly slower than readaccess (e.g., with a large latency of 100's of microseconds or evenmilliseconds) as a consequence of the non-volatile memory storagemechanism within the flash memory bit cells. Flash memory also has writeendurance issues that limit the number of times a given bit cell can bewritten. To facilitate the integration of flash memory into mobiledevices, the industry has evolved a set of standards for isolating someof the non-idealities of flash memory (e.g., poor write endurance,access latency) from the system software (e.g., OS) by integrating aflash management IC into the same chip package as the raw flash memorychips. Managed flash memory (e.g., embedded multimedia card flashstandard, or eMMC) has become the workhorse of the mobile phone businessas it implements such functions as wear leveling (to hide enduranceproblems), error correction, and caching, that allow flash to look morelike an ideal (if slow) memory.

As will be appreciated in light of this disclosure, this increase inflash memory performance presents an opportunity to directly update ahigh resolution e-reader EPD by streaming the multi-frame data directlyfrom flash memory. However, there are some non-trivial issues thatremain. In more detail, and with reference to FIG. 2, a relativelysimple implementation of direct-from-flash EPD update would relocate theinput and output buffers of the conventional software EPD algorithmsfrom DRAM 201 to flash memory 203. If the operating system needs forDRAM 201 can be similarly reduced, the opportunity exists to eliminatethe DRAM 201 entirely from the system, saving substantial cost andremoving the dominant drain on battery power. While advantageous in someregards, there is a strong downside to such a flash-only architecture.Since flash memory is substantially asymmetric on reads and writes, thesimple implementation with reduced/eliminated RAM will generate suchsubstantial flash write traffic that a flash based system would have anunacceptably long response time to user input.

As an example, an 800×600 EPD display has 480,000 pixels that arerequired to be refreshed at 85 Hz when the EPD image is changed inresponse to a button click. Including the typical requisite horizontaland vertical video blanking for this type display, the flash memorywould need to supply data at a bandwidth of approximately 14 Mbytes persecond, which is well within the capability of available managed flash(eMMC) devices. However, for a 40 frame transition (about 470 ms,typical of an e-reader page turn), a conventional architecture willrequire a 40 frame data structure of (480,000 pixels)*(2 bits perpixel)*(40 frames)=4.8 Mbytes be constructed in the flash by the updateprocess. While the size and write bandwidth of this data structure isnot a challenge for the comparatively rapid read/write of typical DRAM201, a flash memory that is interleaving incremental source image readrequests and output write requests will cause substantial slow down(e.g., more than a few seconds with typical eMMC performance) during thepreparation of the 4.8 Mbytes of update frame buffers. This delay iswould manifest as a user interface delay in many cases and isconsequently unsatisfactory for e-reader products.

In contrast to the low performance brute force implementation describedabove, the present disclosure adopts an intermediate frame updaterepresentation that balances the read and write demands to more closelymatch and exploit the characteristics of the flash memory 302. A compactintermediate representation for the frame update is adopted that encodesthe pixel driving waveforms into a series of multi-frame passes, whereinthe update for a given pixel can be collectively encoded in themulti-frame passes in a relatively small number of bits to minimize boththe necessary read and write buffer sizes and bandwidth. To this end,the controller 301 is configured to execute the methodologies providedwith reference to FIGS. 4-6 a-c, in conjunction with flash memory 302and SoC 300. As will be appreciated in light of this disclosure, thecontroller 301 may be implemented, for example, with a fieldprogrammable gate array (FPGA) or other purpose-built hardware that isable to work without external RAM-based frame buffer(s) used in thecomparative configurations shown in FIGS. 1 and 2. For instance, thecontroller 301 may be an integrated circuit custom built to carry outthe various functionalities provided herein. Conventional logicprogramming and/or semiconductor fabrication techniques can be used toachieve this aim, as will be further appreciated. Each of SoC 300,controller 301, and flash memory 302 may be further serve additionalpurposes and otherwise be configured to carry other desiredfunctionality, as will be appreciated in light of this disclosure. Forinstance, in addition to the functionality provided herein, SoC 300 mayinclude a processor and implement any number of functions that SoC 100and 200 might implement, and flash memory 302 can be used to store otherdata as well, such as digital content, operating system, and other code.

Methodology

FIG. 4 illustrates a process for initializing the display updatefunction and changing an image on the EPD 303, in accordance with anembodiment. As can be seen, the methodology can be executed by an EPDdriving system and generally includes five main phases or functions,including initialize the EPD at 401, activate update process at 402,calculate drive signal at 403, update display at 404, and account forcharge change at 405. Underlying details of each of these phases aredescribed in turn, in accordance with various embodiments, and withfurther reference to the example embodiments of FIGS. 3, 5, and 6 a-c.

In this example case, the SoC 300 first initializes the EPD 303 with aset of initialization operations indicated at 401. Starting from theassumption that the EPD display is in a state that is not known by theEPD driving system, the SoC 300 sends a command to the controller 301 todrive one or more known images to the screen at 410, typically all whiteor all black, to reset the EPD material 303 into an optically knownstate. A charge history buffer is constructed in memory in flash memory302, or alternatively in the on-board memory (e.g., RAM) in thecontroller 301 or SoC 300, and cleared at 411. In addition, at 412, abuffer representing the current image is similarly constructed in flashmemory 302 (or in the on-board memory of controller 301 or SoC 300) andset to a state that matches the clearing pattern applied at 410. At theend of the initialization sub-process 401, the charge history buffer isclear and the current image state is set both on the EPD 303 and in theflash memory 302 (or in the on-board memory of controller 301 or SoC300).

At this point in the operation, the SoC 300 can enter into a sleep stateat 420 and await a user or system generated trigger 421. As can befurther seen, the EPD driving system is activated at 402 on receipt of atrigger 421 and the SoC 300 enters a wake state at 422 in preparationfor an image update. Entry into the sleep state 420 and wake state 422can be used to selectively power peripherals (e.g., by virtue of thepower management block 306) in preparation for further action (e.g., adisplay update). Further note that the sleep state 420 and wake state422 can be communicated to the controller 301 by the SoC 300, as needed(e.g., to initiate the calculation sub-process).

Calculate Drive Signal

When an image update is requested (e.g., in response to trigger 421, asreported by SoC 300), the controller 301 commences with calculation ofthe target drive signals at 403, wherein the required frame sequences toeffect the image change from current to next image are determined. Thecalculation sub-process 403 begins with the recall and/or constructionin flash memory 302 of the next image at 430, which may optionallyinvolve the composition of images and other content into a new framebuffer. In accordance with one such embodiment, this process at 430results in three input source data structures being stored in the flashmemory 302, including a first data structure representing each pixel ofthe current image, a second data structure representing each pixel ofthe next image, and a third data structure representing the chargehistory that indicates how many positive and negative frames each pixelhas been driven in its operating history. Although flash memory 302 isused in this example case, other embodiments may use a different memoryavailable to the controller 301, if so desired (e.g., onboard memory orsome other external memory). The temperature of the display is measuredat 431, and this temperature measurement is used to select a particularwaveform look-up table (LUT), or a set of LUTs as the case may be, thatwill be used to update the display. Such waveform tables can be tuned atthe factory under varying temperature conditions to capture the responseof the particular EPD material to electrical drive signals. In addition,waveform LUTs may be selected based on the desired transition type(e.g., fast direct transition with no flashing). In accordance with anembodiment, the waveform LUT is written by the SoC 300 into a small RAMtable in the controller 301. In other embodiments, the waveform LUT canbe stored externally to the controller 301, and accessed via thecontroller 301 using a suitable communication bus.

As can be seen at 432 in the example embodiment of FIG. 4, each of theinput source data structures created at 430, representing the currentimage, the next image and the charge history, are scanned incrementally(from pixel to pixel) from flash memory 302 to form a waveform LUT indexfor each pixel. As can further be seen at 432, each waveform LUT indexis then used to look-up a sequence of low bit count pulse widthmodulation (PWM) indices, which at 433 are subsequently packed and savedinto a set of playback files stored in the flash memory 302. Furthernote in this example embodiment that there is one playback file perpixel per pass, and there is a maximum frame count per duration as well,as will be further explained with reference to FIGS. 6a-c . The numberof passes to transition the display from the current image to the nextimage can vary from one embodiment to the next. Thus, and as indicatedat 434, the scanning/look-up at 432 and pack/saving at 433 can berepeated as needed for each pass. This scanning process (432, 433, 434)can be carried out in its entirety by the controller 301, in accordancewith an embodiment. Variations on the methodology will be apparent. Inone embodiment, the input source data structures are scanned multipletimes, once for each output pass as shown at 432, 433 and 434. In analternative embodiment, the input source data structures are scannedonce and multiple output files (one for each pass) are created inparallel, using buffer memories in controller 301 to pack intermediateresults for writing to flash memory 302. In either case, the resultingpacked PWM index files can be arranged preferentially to allow fastrecall from the flash memory 302, in accordance with an embodiment andas will be appreciated in light of this disclosure.

An example of this sub-process 403 for calculating the EPD update framesis shown in FIG. 5, in accordance with one embodiment. As can be seen,three input bitmap data structures are scanned sequentially—a bitmapmemory buffer of the current image 500 (shown with a 4-bit per pixelrepresentation of a letter “L”), a bitmap memory buffer of the desirednext image 501 (shown with a 4-bit per pixel representation of theletter “o”), and a charge history buffer 503. In this exampleconfiguration, the charge history buffer 503 contains an 8-bit value perpixel that indicates how many positive and negative frames thecorresponding pixel has been driven in its operating history. An 8-bitvalue can, in accordance with one example embodiment, use a 2'scomplement representation to encode values from −128 to +127 frames ofhistorical DC imbalance. As previously explained with reference to 432through 434 of FIG. 4, the three input bitmap data structures 500, 501,and 503 can be scanned from flash 302 by controller 301 to identify anindex value that is in turn used to look-up a set of PWM indices (onePWM index per pass in the multi-pass display update process). Continuingwith the example shown in FIG. 5, the three per-pixel input data sourcesare accessed sequentially. In particular, in the targeted pixel positionof FIG. 5 (as indicated with boxes in each bitmap), the current image500 (the “L”) has a value of 3, the next (desired) image 501 pixel has avalue of 9, and the charge history buffer 503 has a value of hex 0x46.

As can be further seen in this example embodiment of FIG. 5, the threedata values per pixel, depicted in binary at 504, 505, and 506, areconcatenated together to form a look-up index 507 into the waveform LUT508. The LUT 508 may be implemented, for example, with RAM withincontroller 301. In order to reduce the size of the waveform LUT 508, theconcatenated look-up index 507 contains only a few of the mostsignificant bits (MSBs) of the charge history buffer value 504, inaccordance with an embodiment. In this particular example, the top threeMSBs of the charge history buffer value 504 are used for concatenation507. So, 505 shows the binary value (0011) of the target pixel from thecurrent image 500, 506 shows the binary value (1001) of that targetpixel from the next image 501, and 505 shows a portion of the binaryvalue (010; top three MSBs) of the target pixel from the charge history503. In this way, a look-up index 507 is formed that containsinformation about the corresponding pixel, including charge history, thecurrent state, and the desired next state. In other embodiments, thecharge history is optional wherein the look-up index is derived fromonly the current and next images 500 and 501, respectively, oralternatively may contribute fewer or more bits to the look-up index507.

As can be further seen in the specific example of FIG. 5, the indexformation process results in a concatenated look-up value of hex 0x239for this particular pixel. Further note in this example embodiment thatthe waveform LUT 508 will have 2048 entries to handle all combinationsof the 11-bit look-up index. Specifically, four bits from the targetpixel of current image 500, plus four bits from that same pixel of nextimage 501, plus three bits from the charge history buffer 503 for thatsame pixel, equals eleven bits total, which in turn allows for 2048distinct entries in waveform LUT 508. Further note in the exampleembodiment depicted in FIG. 5 that each entry of the LUT 508 contains asequence of low bit count PWM indices 509. Continuing with the examplecase, a look-up value of hex 0x239 corresponds to an entry in LUT 508having PWM indices 2, 4, 5 (depicted at 509). As will be appreciated inlight of this disclosure, and with reference FIG. 6a , each PWM index of509 corresponds to a corresponding PWM LUT 607 a-c.

In more detail, and as shown in the example embodiment depicted in FIG.6a , each waveform LUT entry 508 contains three 3-bit values (for atotal of 9 bits) of packed PWM indices. These 3-bit indices represent adiscrete set of polarities and/or durations of the electrical drivesignal to be applied to a given pixel over the course of a multi-passupdate. Thus, the resulting waveform LUT index computed at 432 is usedto access the LUT 508 to identify the corresponding PWM indices 509,which at 433 are then packed or otherwise stored by the controller 301in flash 302 as a set of playback files. There is one playback file perpixel per pass, according to an embodiment, where each PWM indexcorresponds to one pass, so the process can be repeated for each pass aspreviously explained with respect to 434.

FIG. 6a shows one specific example embodiment where there are three PWMindices 509 recalled from the waveform LUT 508, and which correspond tothe PWM index 600 (i.e., PWM indices 2, 4, and 5 identified at 509correspond to indices 2, 4, and 5, respectively, of the PWM index 600).To this end, the three PWM indices collectively encode a three passframe update process, and where the first pass 601 allows eightdifferent duration/polarity combinations ranging from negative polarityfor 24 frames (corresponding to a PWM index 600 value of 0) to positivepolarity for 24 frames (corresponding to a PWM index 600 value of 7). Ascan be further seen in FIG. 6a , two additional passes (602 and 603)have potentially different encodings and ranges. So, with furtherreference to the example shown in FIGS. 5 and 6 a, the result of thewaveform LUT 508 look-up of hex value 0x239 results in a multi-passtriplet of {2,4,5}, which in turn encodes a polarity sequence of {-16,10 and 3} for the three passes as highlighted in FIG. 6a . As previouslyexplained with respect to the calculate drive signal process 403 of FIG.4, the three PWM indices 509 {2,4,5} identified by the look-up at 432for the given pixel, along with the three PWM indices 509 for each ofthe other pixels, can be efficiently packed or otherwise saved intoplayback files at 433, one file per pass, along with the maximumduration or frame count for each pass (over all pixels).

In the example embodiment shown in FIG. 6a , the multi-pass waveformupdate process includes three passes and is constructed to progress froma coarse and large amplitude table 607 a as shown in the first pass 601to a set of progressively finer and smaller amplitude tables (607 b and607 c) in the second and third passes (602 and 603). As will beappreciated in light of this disclosure, the evolution from coarse grainto fine grain supports driving the EPD pixel hard into saturation (blackor white) in the first pass, which only requires coarse precision, andthen setting the desired final gray shade in the later passes withhigher precision given by fine grain steps. In this embodiment, acompact encoding (9 bits total=512 possible waveform combinations) canrepresent a wide range of transitions from a given gray level to anotheras well as give a wide range of charge correction (positive, negative,of varying amplitudes) to incrementally restore charge balance to thedisplay.

The selection of the values of these encodings can be dependent on manyfactors, including but not limited to the temperature of the display,the desired optical effect (e.g., low ghosting on white background,better grayscale accuracy near black, and other such effects), thedesired response time, the desired charge corrective ability and range,and other pertinent factors. As will be further appreciated in light ofthis disclosure, these considerations and optimizations can be capturedin the combination of the waveform LUT 508 and the PWM LUTs 607, ofwhich there are three in this example case (607 a-c), each PWM LUT 607having eight entries. To this end, note that LUTs 508 and 607 a-c can bepreferentially determined at or before manufacturing time based onempirical measurements of the optical response of the EPD material tothe possible electrical waveform patterns to achieve a high qualityimage rendering on the EPD, in accordance with an embodiment. In otherembodiments, LUTs 508 and 607 a-c can be determined based on theoreticalperformance of the optical response of the EPD material to the possibleelectrical waveform patterns to achieve a high quality image renderingon the EPD, or a combination of such theoretical performance andempirical measurements, as will be appreciated.

Further note that the compact encodings of the PWM LUTs 607 a-c may bebipolar or unipolar in each respective pass and/or may have more orfewer bits per pass; the generality of the present disclosure is notdependent on the particular choice of LUT contents or bit depths.Similarly the scanning order of the input or output data sources neednot proceed sequentially to implement an embodiment of the presentdisclosure; such general transpositions and re-orderings do not alterthe scope of the present disclosure.

As will be further appreciated in light of this disclosure, the compactintermediate PWM encoding of the present disclosure (in this exampleembodiment, 3 bits per pixel per pass) creates a major reduction in thewrite bandwidth when constructing the update. In particular, instead of2 bits per pixel to encode +V, −V or 0V per pixel with, for example, 40unique frames in a conventional update construction method (totaling 80bits per pixel to be stored and read back from memory), a 3 bit perpixel representation of three passes requires only 9 bits per pixel toencode the entire transition. This more compact data structure (on theorder of 1/10^(th) the size, depending on the update length) results ina significant speed-up in the waveform compilation process since thenumber of writes to flash memory 302 (which dominate the execution time)are substantially reduced.

One consequence of a compact intermediate representation of PWM encodingaccording to an embodiment is that the playback time read bandwidthdemands on the flash memory 302 are increased, in this example by afactor of 50% (from 2-bpp to 3-bpp). Due to the asymmetric performanceof flash memory to reads and writes (with reads being substantiallyfaster), this increase in performance is readily handled by conventionalflash memory architectures. By minimizing the write bandwidthrequirements without bloating the read bandwidth requirements out of therange of available flash memories, the architecture of the presentdisclosure makes it possible to manufacture a very low cost and powerefficient EPD e-reader that can be directly run from flash memorywithout a need for large buffer RAMs in either the processor of the SoC300 (or elsewhere in SoC 300), the controller 301, or elsewhere in thedisplay driver system.

Maximum Frame Count.

During the calculation sub-process 403, the maximum frame count of theselected PWM indices (e.g., up to 24 frames in the table shown in FIG.6a for the first pass 601) are recorded for each pass for all pixels onthe display. In total, in this example embodiment, three maximum framecounts are recorded, one for each pass; for the purposes of furtheringthe example of FIGS. 5 and 6 a-c, assume the three maximum passdurations are 19 frames, 18 frames, and 8 frames for the first (601),second (602), and third (603) passes, respectively. As will beappreciated, the maximum frame counts will vary, depending on factorssuch as the degree of difference between the current and next image forany given pixel of the display.

Update Display with Computed Drive Signal

Now, with further reference back to FIG. 4, after the calculationsub-process 403 is carried out, the display can then be updated as shownat sub-process 404. In particular, the sub-process proceeds at 440 toreplay the PWM index file for the maximum PWM length number of framesfor a given pass, and loops until all passes are completed, as indicatedat 441. In more detail, and with further reference to the exampleembodiment of FIGS. 5 and 6 a-c, for each pass the corresponding PWMindex file 509 saved to flash memory 302 at 433 is repeatedly scanned inits entirety, thereby generating a stream of 3-bpp look-up values thatare then used to determine, with a frame counter, whether a given pixelshould be driven with +V, −V or 0V for a given frame. In this exampleembodiment, note that the non-zero pulse width values are drivenimmediately in the first frame. So, for example, if the PWM index 509looks up a negative polarity duration of 16 frames as shown in firstpast 601, the system outputs a −V value for that pixel for 16 frames,starting with the first frame of the first pass 601, as shown in FIG. 6b. In addition, for a given pixel whose PWM duration is less than themaximum PWM duration (referred as Max Frame Count in FIG. 6b ) of thatpass (across all pixels), further note that the pixel is driven to 0Vfor the remaining frames of the first pass which causes the EPD materialto arrest its optical transition. So, with reference to the exampleshown in FIG. 6b , the maximum frame count for the first pass 601 is 19frames, and the subject pixel is driven for 16 frames at −V and at 0Vfor the remaining 3 frames. Once all of the pixels in the array havebeen driven with the appropriate width +V or −V signals for that pass,the system will advance to processing the next pass, and so on, untilall passes are concluded. To this end, the maximum frame count for thesecond pass 602 is 18 frames, and the subject pixel is driven for 10frames at +V and at 0V for the remaining 8 frames. As can be furtherseen, the maximum frame count for the third pass 603 is 8 frames, andthe subject pixel is driven for 3 frames at +V and at 0V for theremaining 5 frames. As will be appreciated, the polarities provided inthe examples include +V, −V and 0V, but other embodiments may includepolarities around some other neutral value which need not necessarily be0V.

Variations will be apparent in light of this disclosure. For instance,in an alternative embodiment, the driving process may defer the drivingof a pixel to later in the pass if the pixel pulse width has a shorterduration than a current duration of the pass (so as to sequentiallydrive pixels in an ordered fashion based on pixel pulse widths orderedfrom longest to shortest duration). In another alternative embodiment,the update waveform may be padded with fixed prefix or suffix frames to,for example, achieve a more uniform driving experience by “rattling” theEPD between +V and −V. In another alternative embodiment, the PWM pulsesmay be selectively dithered within the given pass to smooth thetransitions. In one particular embodiment, the passes are kept as shortas possible to minimize the overall update time (sum of all passes, inframes). Additionally, in one such embodiment and as previouslydiscussed above, the PWM drive commences at time t₀ (start time of agiven pass) or otherwise as soon as possible in the corresponding pass(as shown in FIG. 6b ) so that the EPD transition is as fast (early) aspossible.

With further reference to FIG. 6b , the electrical drive signal 604 forthe example pixel of the multi-pass playback is shown. In this example,the longest duration PWM across all pixels in the first pass 601 is 19frames; thus the controller 301 will scan the entire first playbackbuffer 19 times (once per output frame at, for example, an 85 Hz framerate) until all pixels for that pass 601 have completed their respectivepulse durations as specified by their corresponding 3-bit PWM indexvalues 509 that were looked up from the waveform LUT 508 in thecalculation at 432 and stored in the replay files at 433. Thus, thedashed line of 604, which represents the pixel voltage, of the examplein FIGS. 5 and 6 a-c is driven to −V for 16 frames. As previouslyexplained, in this example embodiment, the subject pixel is driven to 0Vfor the remaining 3 frames of the first pass. Due to the integratingnature of the EPD material, the extra frames of 0V drive appended to theend of the PWM waveform of the first pass 600 can be arbitrarily longwith little effect on the grayscale of the pixel. In some embodiments,if an entire pass is known to have values that correspond to zero lengthPWM values, the particular pass can be preferentially skipped (i.e., noreads of the playback file for that pass) to minimize the overall EPDupdate duration.

With further reference to the example of FIGS. 5 and 6 a-c, the secondpass 602 has a maximum duration or frame count across all pixels of 18frames; the particular pixel shown has a positive duration of 10 framesduring this 18 frame pass. On the last (third) pass 603, the maximumframe count shown in this example is 8 frames (again, across all pixelson the display) and this particular pixel has a 3 frame duration.Overall, the multi-pass duration for this example is 19+18+8=45 frames.

So in this example case, a multipart drive signal 604 is applied incorresponding multiple passes, wherein the first part of the drivesignal is 19 frames long (for first pass 601), the second part is 18frames long (for second pass 602), and the third part is 8 frames long(for third pass 603). The polarity and duration of that polarityassociated with each portion of the multipart drive signal 604 areindicated in the LUTs 607 a-c. For a given pixel, and for any portion ofthe maximum frame count for a given pass (across all pixels) notindicated to be at the polarity, the polarity can be set to 0V. Numerousvariations will be apparent in light of this disclosure.

An approximate optical response 605 of the EPD material is shown in FIG.6c . The EPD electro-optical response is often non-linear (softsaturating) and asymmetric (i.e., it takes more frames to drive a darkpixel blacker than to drive a white pixel similarly blacker). As can beseen, the optical response starts at an initial condition thatapproximately corresponds to the gray level of the current pixel, whichin this example is a gray level of 3 in a 4-bit gray scale, as show inFIG. 5. The first pass 601 consists of a 16 frame negative driving cyclefor this example pixel, causing the optical response to asymptoticallyapproach black. In the second pass 602, the electrical drive signal is a10 frame positive sequence, causing the optical response to rise fromblack rapidly and settle at a mid-gray level. The final phase or pass603 for this example update of an example pixel is a positive 3 frameupdate that creates a small adjustment towards white to reach the finaldesired gray shade, which in this example is a gray level of 9 in a4-bit gray scale, as show in FIG. 5.

Charge History Updating

With further reference back to FIG. 4, the methodology continues at 405with accounting for charge change per pixel. In the example case shown,this is accomplished at 444 by scanning the current and next images, andperforming a lookup to identify the PWM value per pixel transition forall passes, and then adjust the charge history buffer accordingly. Inmore detail, and with reference to the example pixel drive sequenceshown in FIGS. 5 and 6 a-c, the target pixel was driven a total of 16frames at −V and 13 frames at +V, leading to a cumulative DC imbalanceof −3 frames. This DC effect is accounted for at 405. In this chargeaccounting sub-process 405, the input images (both current image 500 andnext image 501) as well as the charge history buffer 503 are re-scannedand the cumulative DC drive imbalance (for this pixel in this example,−3 frames) is added to that pixel in the charge history buffer 503. Inthis example for this pixel, the charge history buffer 503 started athex 0x46; in sub-process 405 this pixel would be adjusted to hex 0x43(0x46+−0x03=0x43). By recording the drive history of the pixels in an8-bit value and using this information (at least the MSBs) to deriveupdate waveforms in the next image update, a feedback system is formedthat allows a large sequence of image updates (e.g., flipping pages inan e-book) to correct the DC imbalance towards neutral (e.g., 0 framesof imbalance) incrementally.

In some embodiments, the sub-process to account for charge change at 405is done following the display update sub-process 404, since 405 takesnon-zero time and, if placed before the display update 404 may introducelatency. However, a number of alternative embodiments generallyreordering and/or inserting additional steps (e.g., delays,pre-computing likely transitions, caching, and other desired processing)will be apparent in light of this disclosure. For example, theaccounting for charge change at 405 can be performed before the displayupdate 404 or before the calculation sub-process 403. Furthermore, thecalculation sub-process 403 can be performed speculatively before thedevice enters sleep state 420 for one or more common or otherwiseanticipated transitions (e.g., the next page in the book) and cached inflash memory 302 (or other accessible memory). Thus, upon a trigger 421,the system can perform a check to determine whether the trigger 421 isconsistent with a pre-computed and cached calculation 403 and, if so,jump directly to the display update at 404 to minimize the delay fromtrigger 421 to display update for high probability page transitions.Such modifications to the ordering of the methodology functions orphases, the inclusion of new steps, and the addition of new memorystructures for intermediate results are all intended to be within thescope of the present disclosure. In some example embodiments, the chargebuffer adjustment at 405 is performed in a way that the output resultwill saturate or clip on overflow. For example, in the 8-bit chargehistory buffer example shown in FIGS. 5 and 6 a-c with 2's complementrepresentation of the charge history at 503, any adjustments that wouldresult in a charge buffer value of less than −128 frames would clip at−128 frames; similarly, any adjustments that would result in a chargebuffer value of greater than +127 frames would clip at +127 frames.

A number of additional features and benefits will be apparent in lightof this disclosure. For instance, and in accordance with someembodiments, the set of playback buffers created at 433 can be designedto have substantially fewer bits per pixel than the source informationto better match the read and write bandwidth available with flash memory302 while both calculating the drive signal at 403 and driving thedisplay at 404. For example, in one example embodiment, assume there arethree playback buffers with 3-bpp each that are derived from a 4-bppcurrent image, a 4-bpp next image, and an 8-bpp charge accumulationbuffer (totaling 16-bpp of information input). This compaction from16-bpp of input to three phases of 3-bpp each (9-bpp total write and3-bpp read while driving) has a number of favorable benefits. By keepingthe intermediate playback buffers relatively compact, the writebandwidth limitations of flash do not dominate latency. For example, an800×600 pixel frame size at 3-bpp leads to a playback buffer size of 180kbytes total, which can be written reasonably quickly to currentlyavailable flash memories. By introducing an intermediate format betweeninput sources and output drive in accordance with an embodiment, theread and write bandwidth limitations of the main system memory (in thiscase flash) are effectively rebalanced. A more brute force approach(e.g., for a transition that requires 40 frames, storing 2-bpp for everyframe results in an 80-bpp temporary file) can lead to a writerequirement approximately 9 times larger than that of the presentdisclosure which is poorly matched to flash memory performance due toits slower write capability. During the playback/scanning of these filesduring the display update sub-process at 404, the slightly increasedread bandwidth requirements according to an embodiment (growing to 3-bppversus the 2-bpp of the brute force approach) are well within the readbandwidth limits of present flash memories.

Example Benefits and Advantages

Numerous benefits and advantages will be apparent in light of thisdisclosure. Example such benefits and advantages are provided herein inthe context of one or more embodiments. However, further note that someembodiments may not have all or any such benefits and advantages, andstill fall within the scope and spirit of this disclosure. To this end,and in accordance with an embodiment, the techniques provided herein canbe used to provide an EPD driving system that tracks the per-pixelcharge history of the EPD material to preserve DC charge balance, overthe life of the display. In an embodiment, the techniques providedherein can be used to provide an EPD driving system that compiles thecurrent image, the desired next image, and the charge history bufferinto a series of compact files for future replay to an EPD. Anotherexample embodiment provides a method for compiling a previous image, anew image and a charge history buffer into a series of compact filesusing a look-up table that represents the electro-optic behavior of theEPD from a given batch (of electronic ink, and the corresponding displaymanufacturing process) and/or at a given temperature. As will be furtherappreciated, the techniques provided herein can be further used toprovide an EPD driving system that reduces the bandwidth requirements ofa display controller's main memory, in accordance with an embodiment.The techniques may be useful, for instance, in implementing relativelylow cost EPD readers and other EPD devices. For example, the electricalcomponent costs may be approximately cut in half compared to a typicalAndroid or Linux based EPD reader. An additional benefit may be thatpower consumption of an EPD-based device configured in accordance withan embodiment is substantially reduced (e.g., up to 80%), therebyenabling new designs that harvest ambient energy (e.g., indoor solarpower) and do not require conventional charging ports, cables, oradapters.

In general, prior EPD-based e-readers use random access memory (RAM)based systems and smartphone-like operating systems to control the EPDdisplay. For instance, one example configuration employs a softwarecontroller and an Android-based operating system which has the downsideof requiring external double data rate dynamic random-access memory (DDRDRAM) for main memory and EPD updates. The implications of this externalDRAM and heavyweight OS is that the EPD reader cannot get into a deepsleep state without having a very impactful wait (e.g., 30 sec˜1 min toboot the system from deep sleep). As a result, despite the zero-powernature of electrophoretic displays, such EPD readers allocate around 75%of their total battery power to an idle state that is spent keeping theoperating system awake enough to be sufficiently responsive to the user.To this end, an embodiment of the present disclosure eliminates the needfor external DRAM from the EPD update process, trades a software-basedEPD controller (typically provided by an electronic ink supplier) for apurpose-built hardware based controller (e.g., field programmable gatearray, or FPGA), which in turn offloads processor burden therebyallowing for a lower cost central processor. For instance, rather thanuse an 800 MHz Cortex-A8 processor running Linux/Android in an EPD-basedreader, a much lower power and lower cost 32 MHz Cortex-M3 can be used(with less than 1% of the compute horsepower).

Thus, an embodiment according to the present disclosure includes amethod for compiling an image update in FPGA logic that is able to workwithout a RAM-based frame buffer(s). Rather, the frame buffer(s), whichare generally required for EPD grayscale operation, are instead storedin relatively lower cost flash memory (e.g., eMMC, aka managed NANDflash memory, or other comparable memory). As will be furtherappreciated in light of this disclosure, the use of flash memory for aframe buffer implies some unusual constraints due to the unequalread/write bandwidths of flash (reads are often 5× to 10× faster thanwrites). The present disclosure provides a method, according to onespecific example embodiment, to first compile the desired image changeinto a set of relatively compact files in an eMMC flash memory (writingeach once), then secondly updating the display by replaying these filesmultiple times each as required (one file is played back to the EPD peroutput frame at somewhere between 10 ms and 20 ms per frame, at 85 Hz)until the sequences complete and the image update is accomplished (e.g.,around 500 ms in total). Compared to a RAM-based EPD system, thebandwidth requirements to memory are substantially reduced according toan embodiment of the present disclosure, such that RAM can be eliminatedand flash used instead, thereby saving power, cost, and board area, aswell as reducing software complexity.

With over a billion smartphones deployed and a growth/replacement cyclethat shows no signs of abating, the prevalence of mobile phones andcloud platforms with significant computing capabilities has opened newopportunities to craft hardware devices that can leverage theseresources, as will be appreciated in light of this disclosure. Theso-called Internet-of-things and wearable technologies are twocategories of devices that either directly or indirectly rely on theaverage user having a smartphone nearby to control, configure, monitor,and interact with cloud services linked to such accessory devices. It isin this spirit and context that the present disclosure is provided.

Numerous variations and configurations will be apparent in light of thisdisclosure. For example, one example embodiment of the presentdisclosure provides a display driving system. The system includes amemory and a controller. The memory is configured to store a firstmemory structure containing a representation of a current optical stateof a display, a second memory structure containing a representation of anext optical state of the display, and a third memory structurecontaining a look-up table that encodes each of a plurality of sets ofelectrical drive signals in an intermediate representation. Thecontroller is configured to combine content of the first memorystructure with content of the second memory structure to form an index,use the index to look-up an intermediate representation of a target setof electrical drive signals in the look-up table, and drive the displaybased at least in part on the target set of electrical drive signals,thereby updating the display from the current optical state to the nextoptical state. In some cases, the memory is further configured to storean additional memory structure containing a cumulative charge history ofdrive signals applied to the display. In one such case, the controlleris further configured to use at least partial content of the additionalmemory structure to form the index. In another such case, the controlleris further configured to update the additional memory structure eachtime the display is updated, so as to maintain the cumulative chargehistory. In some cases, the target set of electrical drive signals formsa multipart drive signal, and the intermediate representation of thetarget set of electrical drive signals includes a plurality of look-uptable indices, each look-up table index indicating a polarity andduration for a distinct part of the multipart drive signal to be appliedto the display in corresponding distinct passes, thereby providing amulti-pass display update process. In one such case, the multi-passdisplay update process includes three passes, each distinct passcorresponding to a distinct part of the multipart drive signal. In somecases, the controller is configured to form an index for each of atleast multiple pixels of the display, and to use each of those indicesto look-up a corresponding intermediate representation of a target setof electrical drive signals for the associated pixel, and to drive thedisplay based at least in part on each target set of electrical drivesignals. In some cases, the memory comprises flash memory. In somecases, the display is an electrophoretic display. Variations will beapparent. For instance, another embodiment provides a mobile computingdevice that includes the display driving system as variously defined inthis paragraph. In some such cases, the mobile computing device is ane-reader or tablet computer.

Another example embodiment of the present disclosure provides anelectrophoretic display driving system. The system includes a flashmemory and a controller. The flash memory is configured to store a firstmemory structure containing a representation of a current optical stateof a display, a second memory structure containing a representation of anext optical state of the display, and a third memory structurecontaining a look-up table that encodes each of a plurality of multipartdrive signals in an intermediate representation. The controller isconfigured to combine content of the first and second memory structuresto form an index, and use the index to look-up an intermediaterepresentation of a target multipart drive signal in the look-up table,wherein the intermediate representation includes a plurality of look-uptable indices, each look-up table index associated with a polarity andduration for a distinct part of the target multipart drive signal to beapplied to the display in corresponding distinct passes, therebyproviding a multi-pass display update process. The controller is furtherconfigured to drive the display based at least in part on the targetmultipart drive signal, thereby updating the display from the currentoptical state to the next optical state. In some cases, the controlleris configured to form an index for each of at least multiple pixels ofthe display, and to use each of those indices to look-up a correspondingintermediate representation of a target multipart drive signal for theassociated pixel, and to drive the display based at least in part oneach target multipart drive signal. Another example embodiment providesa mobile computing device (e.g., tablet, e-reader, smartphone) thatincludes the display driving system as variously defined in thisparagraph.

Another example embodiment of the present disclosure provides anelectrophoretic display driving system. The system includes a flashmemory and a controller. The flash memory is configured to store a firstmemory structure containing a representation of a current optical stateof a display, a second memory structure containing a representation of anext optical state of the display, a third memory structure containing acumulative charge history of drive signals applied to the display, and afourth memory structure containing a look-up table that encodes a set ofelectrical drive signals in an intermediate representation. Thecontroller is configured to combine content of the first, second, andthird memory structures to form an index for accessing the look-uptable, and use the index to look-up the intermediate representation ofthe set of electrical drive signals in the look-up table, wherein theintermediate representation comprises a plurality of look-up tableindices, each look-up table index associated with drive signal polarityand duration to be applied to the display in distinct passes, therebyproviding a multi-pass display update process. The controller is furtherconfigured to drive the display based at least in part on the set ofelectrical drive signals, thereby updating the display from the currentoptical state to the next optical state. In some cases, the controlleris further configured to update the third memory structure each time thedisplay is updated, so as to maintain the cumulative charge history. Insome cases, the controller is configured to form an index for each of atleast multiple pixels of the display, and to use each of those indicesto look-up a corresponding intermediate representation of a set ofelectrical drive signals for the associated pixel, and to drive thedisplay based at least in part on each set of electrical drive signals.In some cases, each pass of the multi-pass display update process isassociated with a maximum frame count with respect to all pixels, and inresponse to the drive signal duration for a given pixel and a given passbeing less than the maximum frame count, the controller is furtherconfigured to set the drive signal polarity to a neutral value for anyremaining frames of that pass until the maximum frame count is reached.In some cases, the controller is further configured to commence thedrive signal polarity at the beginning of a given pass rather than laterin a given pass. Another example embodiment provides a mobile computingdevice that includes the display driving system as variously defined inthis paragraph.

The foregoing description of the embodiments of the disclosure has beenpresented for the purposes of illustration and description. It is notintended to be exhaustive or to limit the present disclosure to theprecise form disclosed. Many modifications and variations are possiblein light of this disclosure. It is intended that the scope of thepresent disclosure be limited not by this detailed description, butrather by the claims appended hereto.

What is claimed is:
 1. A display driving system, comprising: a memoryconfigured to store: a first memory structure containing arepresentation of a current optical state of a display; a second memorystructure containing a representation of a next optical state of thedisplay; and a third memory structure containing a look-up table thatencodes each of a plurality of sets of electrical drive signals in anintermediate representation; and a controller configured to: combinecontent of the first memory structure with content of the second memorystructure to form an index; use the index to look-up an intermediaterepresentation of a target set of electrical drive signals in thelook-up table; and drive the display based at least in part on thetarget set of electrical drive signals, thereby updating the displayfrom the current optical state to the next optical state.
 2. The displaydriving system of claim 1 wherein the memory is further configured tostore an additional memory structure containing a cumulative chargehistory of drive signals applied to the display.
 3. The display drivingsystem of claim 2 wherein the controller is further configured to use atleast partial content of the additional memory structure to form theindex.
 4. The display driving system of claim 2 wherein the controlleris further configured to update the additional memory structure eachtime the display is updated, so as to maintain the cumulative chargehistory.
 5. The display driving system of claim 1 wherein the target setof electrical drive signals forms a multipart drive signal, and theintermediate representation of the target set of electrical drivesignals comprises a plurality of look-up table indices, each look-uptable index indicating a polarity and duration for a distinct part ofthe multipart drive signal to be applied to the display in correspondingdistinct passes, thereby providing a multi-pass display update process.6. The display driving system of claim 5 wherein the multi-pass displayupdate process includes three passes, each distinct pass correspondingto a distinct part of the multipart drive signal.
 7. The display drivingsystem of claim 1 wherein the controller is configured to form an indexfor each of at least multiple pixels of the display, and to use each ofthose indices to look-up a corresponding intermediate representation ofa target set of electrical drive signals for the associated pixel, andto drive the display based at least in part on each target set ofelectrical drive signals.
 8. The display driving system of claim 1wherein the memory comprises flash memory.
 9. The display driving systemof claim 1 wherein the display is an electrophoretic display.
 10. Amobile computing device comprising the display driving system ofclaim
 1. 11. The mobile computing device of claim 10 wherein the mobilecomputing device is an e-reader or tablet computer.
 12. Anelectrophoretic display driving system, comprising: a flash memoryconfigured to store: a first memory structure containing arepresentation of a current optical state of a display; a second memorystructure containing a representation of a next optical state of thedisplay; and a third memory structure containing a look-up table thatencodes each of a plurality of multipart drive signals in anintermediate representation; and a controller configured to: combinecontent of the first and second memory structures to form an index; usethe index to look-up an intermediate representation of a targetmultipart drive signal in the look-up table, wherein the intermediaterepresentation comprises a plurality of look-up table indices, eachlook-up table index associated with a polarity and duration for adistinct part of the target multipart drive signal to be applied to thedisplay in corresponding distinct passes, thereby providing a multi-passdisplay update process; and drive the display based at least in part onthe target multipart drive signal, thereby updating the display from thecurrent optical state to the next optical state.
 13. The display drivingsystem of claim 12 wherein the controller is configured to form an indexfor each of at least multiple pixels of the display, and to use each ofthose indices to look-up a corresponding intermediate representation ofa target multipart drive signal for the associated pixel, and to drivethe display based at least in part on each target multipart drivesignal.
 14. A mobile computing device comprising the display drivingsystem of claim
 12. 15. An electrophoretic display driving system,comprising: a flash memory configured to store: a first memory structurecontaining a representation of a current optical state of a display; asecond memory structure containing a representation of a next opticalstate of the display; a third memory structure containing a cumulativecharge history of drive signals applied to the display; and a fourthmemory structure containing a look-up table that encodes a set ofelectrical drive signals in an intermediate representation; and acontroller configured to: combine content of the first, second, andthird memory structures to form an index for accessing the look-uptable; use the index to look-up the intermediate representation of theset of electrical drive signals in the look-up table, wherein theintermediate representation comprises a plurality of look-up tableindices, each look-up table index associated with drive signal polarityand duration to be applied to the display in distinct passes, therebyproviding a multi-pass display update process; and drive the displaybased at least in part on the set of electrical drive signals, therebyupdating the display from the current optical state to the next opticalstate.
 16. The display driving system of claim 15 wherein the controlleris further configured to update the third memory structure each time thedisplay is updated, so as to maintain the cumulative charge history. 17.The display driving system of claim 15 wherein the controller isconfigured to form an index for each of at least multiple pixels of thedisplay, and to use each of those indices to look-up a correspondingintermediate representation of a set of electrical drive signals for theassociated pixel, and to drive the display based at least in part oneach set of electrical drive signals.
 18. The display driving system ofclaim 15 wherein each pass of the multi-pass display update process isassociated with a maximum frame count with respect to all pixels, and inresponse to the drive signal duration for a given pixel and a given passbeing less than the maximum frame count, the controller is furtherconfigured to set the drive signal polarity to a neutral value for anyremaining frames of that pass until the maximum frame count is reached.19. The display driving system of claim 15 wherein the controller isfurther configured to commence the drive signal polarity at thebeginning of a given pass rather than later in a given pass.
 20. Amobile computing device comprising the display driving system of claim15.